A casting method known as "compression casting" has been widely used to form a casting with a dense and uniform structure, without internal structural defects, such as blow holes, and with improved mechanical properties. Typically, when metal is subjected to compression casting, as the temperature of the molten metal decreases, the metal solidifies and increases in density. Conventional compression casting methods, however, tend to produce internal structural defects, and, in particular, voids or blow holes, when the molten metal solidifies at an insufficient rate relative to a rate of drop in temperature. It is necessary to compress the molten metal sufficiently and properly in the casting cavity to permit the solidification of molten metal without the production of internal structural defects in the casting.
In compression casting, as is common in die casting, it is typical to compress molten metal in the casting cavity at a high pressure ranging between about 1,000 and 2,000 atmospheres (atms.). In order for the casting cavity resist such high pressures, metallic molds usually must be used to form the casting cavity.
In recent years, improvements in casting technology have made it possible to form a casting with no blow holes, even when a low compression pressure, such as about 1,000 atms., is used. Because of such improvements, some castings, without structural defects, can be formed with compression pressures sufficiently low so that even a sand mold can be used. For instance, as is known from Japanese Unexamined Patent Publication No. 63-137564, a sand mold, such as one made of formed casting sand, is used in compression casting. This sand mold is, after being filled with a molten metal in its cavity, compressed with a high-pressure gas in a gas chamber.
There is, however, a drawback to the conventional use of a metal or sand casting mold in compression casting. In particular, in die casting, in which a metal mold having a core is used, the metal mold typically has a pouring gate remote from its casting cavity. Therefore, a substantial loss in compression pressure applied to molten metal in the casting cavity is caused. In particular, when a metal mold with a casting cavity which is complicated in configuration, and hence, which has a large surface area, is used, the metal mold has a large heat-dissipation area. Consequently, the molten metal in the casting cavity, and, in particular, in intricate and deep sections of the cavity, tends to solidify at an early stage, so that it is difficult to exert a sufficient compression pressure on the molten metal in such sections before the metal solidifies. Since a high compression pressure must be applied to the molten metal in order to prevent formation of voids in the casting, a relatively large compression device, to exert sufficient compression pressure, is required. Thus, the risk of damaging or deforming the core of the die casting mold is brought about.
On the other hand, if a sand mold is used, a large high-pressure gas chamber with a door is also required. When such a high-pressure gas chamber is used, however, it is difficult to easily manage pouring or feeding molten metal into the casting cavity and closing the door for applying and maintaining high compression pressure. This results in inefficient casting and low productivity. Furthermore, high compression pressure has been found to adversely affect the desired close contact of the molten metal to the surface of the casting cavity. Accordingly, the molten metal solidifies slowly, resulting in a rough casting structure and poor mechanical properties. Additionally, an ill-timed or delayed application of compression pressure, after the molten metal has been completely fed or poured into a casting cavity having a complicated configuration, brings about an early partial solidification of the molten metal, particularly in intricate and deep sections of the casting cavity. Thus, it is difficult to exert a uniform compression pressure over the whole area.